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<h1>Isolation Levels and Concurrency</h1>
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<ul class="minitoc">
<li>
<a href="#Isolation+Levels+and+Concurrency">Isolation Levels and Concurrency</a>
</li>
</ul>
<a name="N1003F"></a><a name="Isolation+Levels+and+Concurrency"></a>
<h3>Isolation Levels and Concurrency</h3>
<div style="margin-left: 0 ; border: 2px">
<p>Derby provides four transaction isolation levels. Setting the transaction isolation level for a connection allows a user to specify how severely the user's transaction should be isolated from other transactions. For example, it allows you to specify whether transaction A is allowed to make changes to data that have been viewed by transaction B before transaction B has committed.</p>
<p>A connection determines its own isolation level, so JDBC provides an application with a way to specify a level of transaction isolation. It specifies four levels of transaction isolation. The higher the transaction isolation, the more care is taken to avoid conflicts; avoiding conflicts sometimes means locking out transactions. Lower isolation levels thus allow greater concurrency.</p>
<p>Inserts, updates, and deletes always behave the same no matter what the isolation level is. Only the behavior of select statements varies.</p>
<p>To set isolation levels you can use the JDBC <em>Connection.setIsolationLevel</em> method or the SQL SET ISOLATION statement. The names of the isolation levels are different, depending on whether you use a JDBC method or SQL statement. <a href="#TBLSII-CONCEPTS-ISO_LEVEL">Table 3</a> shows the equivalent names for isolation levels whether they are set through the JDBC method or an SQL statement.<br>
</p>
<p>
<strong>Table 3. Mapping of JDBC transaction isolation levels to Derby isolation levels</strong>
<br>
</p>
<table class="ForrestTable" cellspacing="1" cellpadding="4">
<tr>
<th><a name="WQ11"></a>Isolation levels for JDBC</th><th><a name="WQ12"></a>Isolation levels for SQL</th>
</tr>
<tr>
<td headers="WQ11" width="58%">Connection.TRANSACTION_READ_ UNCOMMITTED (ANSI level 0)  </td><td headers="WQ12" width="42%">UR, DIRTY READ, READ UNCOMMITTED</td>
</tr>
<tr>
<td headers="WQ11" width="58%">Connection.TRANSACTION_READ_COMMITTED (ANSI level 1)  </td><td headers="WQ12" width="42%">CS, CURSOR STABILITY, READ COMMITTED</td>
</tr>
<tr>
<td headers="WQ11" width="58%">Connection.TRANSACTION_REPEATABLE_READ (ANSI level 2)  </td><td headers="WQ12" width="42%">RS</td>
</tr>
<tr>
<td headers="WQ11" width="58%">Connection.TRANSACTION_SERIALIZABLE (ANSI level 3)  </td><td headers="WQ12" width="42%">RR, REPEATABLE READ, SERIALIZABLE</td>
</tr>
</table>
<p>
<a name="IDX681"></a> These levels allow you to avoid particular kinds of transaction anomalies, which are described in <a href="#TBLSII-CONCEPTS-17741">Table 4</a>.<br>
</p>
<p>
<strong>Table 4. Transaction Anomalies</strong>
<br>
</p>
<table class="ForrestTable" cellspacing="1" cellpadding="4">
<tr>
<th><a name="WQ14"></a>Anomaly</th><th><a name="WQ15"></a>Example</th>
</tr>
<tr>
<td headers="WQ14" width="49%">Dirty Reads  <p>A dirty read happens when a transaction reads data that is being modified by another transaction that has not yet committed.</p>
</td><td headers="WQ15" width="52%">Transaction A begins. <pre>UPDATE employee SET salary = 31650
WHERE empno = '000090'
</pre>Transaction B begins. <pre>SELECT * FROM employee
</pre>(Transaction B sees data updated by transaction A. Those updates have not yet been committed.)</td>
</tr>
<tr>
<td headers="WQ14" width="49%">Non-Repeatable Reads  <p>Non-repeatable reads happen when a query returns data that would be different if the query were repeated within the same transaction. Non-repeatable reads can occur when other transactions are modifying data that a transaction is reading.</p>
</td><td headers="WQ15" width="52%">Transaction A begins. <pre>SELECT * FROM employee
WHERE empno = '000090' 
</pre>Transaction B begins. <pre>UPDATE employee SET salary = 30100
WHERE empno = '000090'
</pre>(Transaction B updates rows viewed by transaction A before transaction A commits.) If Transaction A issues the same SELECT statement, the results will be different.</td>
</tr>
<tr>
<td headers="WQ14" width="49%">Phantom Reads  <p>Records that appear in a set being read by another transaction. Phantom reads can occur when other transactions insert rows that would satisfy the WHERE clause of another transaction's statement.</p>
</td><td headers="WQ15" width="52%">Transaction A begins. <pre>SELECT * FROM employee
 WHERE salary &gt; 30000 
</pre>Transaction B begins. <pre>INSERT INTO employee
(empno, firstnme, midinit,
lastname, job,
salary) VALUES ('000350', 'NICK',
'A','GREEN','LEGAL COUNSEL',35000)
</pre>Transaction B inserts a row that would satisfy the query in Transaction A if it were issued again.</td>
</tr>
</table>
<p>The transaction isolation level is a way of specifying whether these transaction anomalies are allowed. The transaction isolation level thus affects the quantity of data locked by a particular transaction. In addition, a DBMS's locking schema might also affect whether these anomalies are allowed. A DBMS can lock either the entire table or only specific rows in order to prevent transaction anomalies.</p>
<p>
<a href="#TBLSII-CONCEPTS-28076">Table 5</a> shows which anomalies are possible under the various locking schemas and isolation levels.<br>
</p>
<p>
<strong>Table 5. When Transaction Anomalies Are Possible</strong>
<br>
</p>
<table class="ForrestTable" cellspacing="1" cellpadding="4">
<tr>
<th><a name="WQ18"></a>Isolation Level</th><th><a name="WQ19"></a>Table-Level Locking</th><th><a name="WQ20"></a>Row-Level Locking</th>
</tr>
<tr>
<td headers="WQ18" width="57%">TRANSACTION_READ_UNCOMMITTED</td><td headers="WQ19" width="21%">Dirty reads, nonrepeatable reads, and phantom reads possible</td><td headers="WQ20" width="22%">Dirty reads, nonrepeatable reads, and phantom reads possible</td>
</tr>
<tr>
<td headers="WQ18" width="57%">TRANSACTION_READ_COMMITTED</td><td headers="WQ19" width="21%">Nonrepeatable reads and phantom reads possible</td><td headers="WQ20" width="22%">Nonrepeatable reads and phantom reads possible</td>
</tr>
<tr>
<td headers="WQ18" width="57%">TRANSACTION_REPEATABLE_READ</td><td headers="WQ19" width="21%">Phantom reads not possible because entire table is locked</td><td headers="WQ20" width="22%">Phantom reads possible</td>
</tr>
<tr>
<td headers="WQ18" width="57%">TRANSACTION_SERIALIZABLE</td><td headers="WQ19" width="21%">None</td><td headers="WQ20" width="22%">None</td>
</tr>
</table>
<p>The following <em>java.sql.Connection</em> isolation levels are supported:</p>
<ul>
<li>
<a name="IDX685"></a> <tt>TRANSACTION_SERIALIZABLE</tt> 
<p>
<tt>RR</tt>, <tt>SERIALIZABLE</tt>, or <tt>REPEATABLE READ</tt> from SQL.</p>
<p>
<em>TRANSACTION_SERIALIZABLE</em> means that Derby treats the transactions as if they occurred serially (one after the other) instead of concurrently. Derby issues locks to prevent all the transaction anomalies listed in <a href="#TBLSII-CONCEPTS-17741">Table 4</a> from occurring. The type of lock it issues is sometimes called a <em>range lock</em>. For more information about range locks, see <a href="develop74.html#HDRCONCEPTS842524">Scope of Locks</a>.</p>
</li>
<li>
<a name="IDX686"></a> <tt>TRANSACTION_REPEATABLE_READ</tt> 
<p>
<tt>RS</tt> from SQL.</p>
<p>
<em>TRANSACTION_REPEATABLE_READ</em> means that Derby issues locks to prevent only dirty reads and non-repeatable reads, but not phantoms. It does not issue range locks for selects.</p>
</li>
<li>
<a name="IDX687"></a> <tt>TRANSACTION_READ_COMMITTED</tt> 
<p>
<tt>CS</tt> or <tt>CURSOR STABILITY</tt> from SQL.</p>
<p>
<em>TRANSACTION_READ_COMMITTED</em> means that Derby issues locks to prevent only dirty reads, not all the transaction anomalies listed in <a href="#TBLSII-CONCEPTS-17741">Table 4</a>.</p>
<p>
<em>TRANSACTION_READ_COMMITTED</em> is the default isolation level for transactions.</p>
</li>
<li>
<a name="IDX688"></a> <tt>TRANSACTION_READ_UNCOMMITTED</tt> 
<p>
<tt>UR</tt>, <tt>DIRTY READ</tt>, or <tt>READ UNCOMMITTED</tt> from SQL.</p>
<p>For a SELECT INTO, FETCH with a read-only cursor, full select used in an INSERT, full select/subquery in an UPDATE/DELETE, or scalar full select (wherever used), READ UNCOMMITTED allows:</p>
<ul>
<li>Any row that is read during the unit of work to be changed by other application processes.</li>
<li>Any row that was changed by another application process to be read even if the change has not been committed by the application process.</li>
</ul>
<p>For other operations, the rules that apply to READ COMMITTED also apply to READ UNCOMMITTED.</p>
</li>
</ul>
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